1. Technical Field
This disclosure generally relates to computer systems, and more specifically relates to compilers that generate executable code for computer systems.
2. Background Art
Computer systems have evolved into extremely sophisticated devices, and computer systems may be found in many different settings. Dramatic advances in both hardware and software (e.g., computer programs) have drastically improved the performance of computer systems. Modern software has become very complex when compared to early computer programs. Many modern computer programs have tens or hundreds of thousands of instructions. The execution time (and hence, performance) of a computer program is very closely related to the number and complexity of instructions that are executed as the computer program runs. Thus, as the size and complexity of computer programs increase, the execution time of the computer program increases as well.
Unlike early computer programs, modern computer programs are typically written in a high-level language that is easy to understand by a human programmer. Special software tools known as compilers take the human-readable form of a computer program, known as “source code”, and convert it into “machine code” or “object code” instructions that may be executed by a computer system. Because a compiler generates the stream of machine code instructions that are eventually executed on a computer system, the manner in which the compiler converts the source code to object code affects the execution time of the computer program.
The execution time of a computer program, especially complex computer programs, is a function of the arrangement and type of instructions within the computer program. Many different optimizations have been developed so the code produced by compilers has better run-time performance. One such optimization is known as “strength reduction”, which relates to replacing one or more expensive instructions with one or more less-expensive instructions. The most common such optimization is to replace repeated multiplications by repeated additions or subtractions. Indeed, this is often the only form of strength reduction performed by an optimizing compiler, since other related opportunities, such as divisions and modulos, are comparatively infrequent.